Missing nuclear materials can pose a significant environmental and health risk through direct exposure, co-mingling in the metal recycling stream, use in contaminated consumer products, and use in terrorist activities. According to International Atomic Energy Agency (IAEA) and Nuclear Regulatory Commission (NRC), hundreds of radioactive sources are abandoned, lost or stolen every year, with a large fraction of the radioactive sources never recovered. Such abandoned, lost and stolen nuclear materials can be transported to commercial and military interests and used with pernicious intentions. Therefore, detection of nuclear materials is an essential deterrent and tool for preventing the unauthorized transport and pernicious use of abandoned, lost or stolen nuclear materials will neither be diverted from controlled facilities, nor transported without authorization.
Every nuclear material emits one or more of four types of radiations—Alpha, Beta, Gamma and Neutron. While Alpha and Beta rays can be easily concealed with minor amount of shielding, the primary and secondary Gamma rays and Neutrons are hard to shield, and therefore are useful signatures for detection by a radiation detector. FIG. 12 shows neutron signatures from weapons and reactor grade fissile materials, namely 239Pu and 235U. As indicated, most of the neutrons emitted from weapons and reactor grade fissile materials fall in the energy range of 0.5 and 3 MeV, which make them “fast” neutrons. “Thermal” neutrons, on the other hand, have a much lower energy (<1 eV). The fast and thermal neutrons have very different mechanisms of interacting with matter, and therefore the respective detectors are made out of different technologies.
Neutron detection is the effective detection of neutrons entering a well-positioned neutron detector. For a neutron detector to work, an incident neutron must be made to interact with the detector material to create enough energy, which when converted to an appropriate electrical or optical signal, would indicate detection. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding, and circuitry for converting optical signals generated during neutron detection into electrical signals. Detection software consists of analysis tools that perform tasks such as graphical analysis of the neutron detection signals to measure the number and energies of neutrons striking the detector.
In the past, although simple neutron counters have been successfully developed and deployed, using proportional chambers filled with He or BF3, often enclosed in moderating materials. However, such detectors are incapable of tracking the neutron by themselves (i.e., determining the incident direction traveled by the neutron to the detector, and hence the location of the neutron's source). Further, the gas including quenching agents needed in conventional neutron counters must be periodically replenished.
A large area detector capable of directional information for fast neutrons is disclosed in “Directional Neutron Detectors For Use with 14 MeV Neutrons”, N. Mascharenhas et. al., Sandia Report SAND 2005-6255 (2005). This detector utilizes compact arrays of scintillator dye doped optical fibers that reconstruct the trajectory of a series of recoil protons. Although this detector exhibits a neutron track angular resolution of about 10 degrees, the neutron detection efficiency is below 1%. For a sensitive detector to be practical over a 1000 cm2 scale, the neutron efficiency would have to be better than 50%.
A double proton recoil time of flight fast neutron detector using two plastic scintillator planes is described in “Calibration and Testing of a Large-Area Fast-Neutron Directional Detector”, P. E. Vanier et. al., BNL-79632-2007-CP (2007), and “Directional detection of fission-spectrum neutrons”, P. E. Vanier et. al., Applications and Technology Conference, LISAT 2007, IEEE Long Island. This detector has adequate area resolution, but the angular resolution is unclear, single neutron counting is not possible, and the set-up is too large to be used for mobile surveillance (i.e., too large and heavy to be transported by a tactical vehicle such as a Humvee or small truck).
What is needed is a neutron detector that is capable of accurately determining both neutron flux and the location of a neutron source, such as abandoned, lost or stolen nuclear material. What is particularly needed is a neutron detector that both accurately determines neutron flux and source location, and is compact (i.e., having a size and weight that make it capable of being mounted on a tactical vehicle for mobile surveillance).